JPS62271528A - Subscriber line transmission equipment - Google Patents

Abstract

PURPOSE:To miniaturize a circuit scale by providing an equalizer and an echo canceller thereafter on the 4-wire reception side of a hybrid circuit so as to form an echo component from transmission information and subtracting the component from an equalizer output. CONSTITUTION:A transmission signal is inputted to a hybrid circuit 4, sent to a transmission line from a 2-wire terminal 3, and inputted to an echo canceller 6, subjected to A/D conversion 7, the same signal as the echo component in the output of the equalizer 5 is generated approximately in an echo filter 8, the result is subjected to D/A conversion 9 and subtracted. A signal from an opposite party is inputted from the terminal 3, the waveform distortion due to the characteristic of the transmission line is equalized by the equalizer 5 to subtract the approximated echo component to output (2) correct reception information. Through the constitution above, since an echo canceller 6 has only to cancel the echo component at the point of time of information identification, the extracting speed of the A/D converter 7 is sufficient with the transmission symbol speed and the number of order of the echo filter 8 is small and the circuit scale is miniaturized.

Description

Detailed Description of the Invention 3. Detailed Description of the Invention (Field of Industrial Application) The present invention is directed to a subscriber line transmission system using a hybrid circuit, in which a four-wire transmission side is connected to a four-wire transmission side due to an imperfection of the hybrid circuit. Related to subscriber line transmission equipment that eliminates echoes to the receiving side and equalizes the waveform distortion of the received signal due to line characteristics, enables correct reception of signals from the other side, and enables error-free bidirectional transmission. It is.

(Prior Art) When performing bidirectional full-duplex communication using a two-line subscriber line, a hybrid circuit is often used in order not to widen the frequency band used. A typical example of this is the current analog telephone, and even in digital subscriber line transmission systems, systems using such hybrid circuits are being widely studied and are being put into practical use. In digital subscriber line transmission systems, since the transmission speed is generally high, the distortion of the received signal waveform due to the loss characteristics of the transmission line becomes large.

In addition, since it is actually impossible to create a hybrid circuit that works perfectly with a variety of subscriber lines, it is difficult to make a hybrid circuit from the four-wire transmitting side to the four-wire receiving side due to impedance mismatch between the hybrid circuit and the subscriber line. produces an echo.

These distortions and echoes become obstacles to correctly receiving signals from the other party, so in subscriber line transmission equipment, it is necessary to equalize the waveform distortion of the received signal due to the line characteristics and to It is necessary to cancel the echo to the receiving side.

Regarding this type of conventional subscriber line transmission equipment, see "IEICE Technical Research Report C380-109 (1980) Jl.
It is described in.

FIG. 2 is a block diagram of a conventional subscriber line transmission device. In the above-mentioned document, although it is expressed divided into detailed components, it is essentially the same as FIG. 2.

The subscriber line transmission device in Fig. 2 has a transmitting terminal 1, a receiving terminal 2,
and a two-wire side terminal 3, and is composed of a no-brid circuit 4, an equalization amount 5, and an echo canceller 6.

The hybrid circuit 4 is a circuit that converts between the four-wire side and the two-wire side. Ideally, the signal from the four-wire transmission side of the hybrid circuit 4 flows to the two-wire side, and It acts so that the signal from the wire flows to the four-wire receiving side. Further, the equalization amount 5 is a circuit for equalizing the waveform distortion of the received signal due to the line characteristics, and the echo canceller 6 cancels the echo from the four-wire transmitting side to the four-wire receiving side of the hybrid circuit 4. This is a useless circuit.

Now, the echo canceller 6 has a high-frequency cutoff characteristic in which the echo from the four-wire transmitting side to the four-wire receiving side of the no-ibrid circuit 4 has a long tail in the time domain, and generates approximately the same signal as this echo. Since the order of the echo filter tends to be large, it is often constructed using a dizotal circuit as shown in FIG. In FIG. 2, an echo canceller 6, an analog signal (digital converter 7 (hereinafter referred to as a 0 converter), an echo filter 8, a digital/analog converter 9 (hereinafter referred to as a D/A converter), and a subtraction circuit A/II) The converter 7 is a circuit for digitizing the transmission signal, and in the digital subscriber line transmission system, since the transmission signal is a digital signal, it is usually a digital signal. Only the circuit is included. The echo filter 8 is a circuit for generating approximately the same signal as the echo from the four-wire transmitting side to the four-wire receiving side, and is usually configured as an FIR type filter represented by a transpersal type filter. The D/A converter 9 is a circuit for converting a digital signal into an analog signal, or for configuring a digital signal and smoothing a base pulse. Further, the subtraction circuit 10 is a circuit that performs signal subtraction, and is a circuit that performs signal subtraction.
By subtracting the approximate echo output from the converter 9 from the signal on the four-wire receiving side, it acts to cancel the echo component.

In FIG. 2, the transmission signal input from the transmission terminal 1 is input from the four-wire transmission side of the no-brid circuit 4, is output from the two-wire side to the two-wire side terminal 3, and is sent out to the transmission line. At this time, this transmission signal is transmitted to the echo canceller 6.
It is also input to the internal converter 7. The φ converter 7 digitizes the transmitted signal and supplies it to the echo filter 8. The echo filter 8 generates approximately the same signal as the echo by approximating the characteristics of the echo path, and converts this into D.
/A converter 9. The D/A converter 9 converts the approximate echo, which is a digital signal, into an analog signal, and supplies this to the subtraction side input of the subtraction circuit 1θ. When , the subtracted side input of the subtraction circuit 10 is connected to the two-wire side terminal 3 from the transmission line.
A received signal is inputted to the two-wire side of the hybrid circuit 4 through the two-wire side, and outputted to the four-wire receiving side. This received signal includes an echo component from the four-wire transmitting side to the four-wire receiving side. The subtraction circuit 10 subtracts the approximate echo converted into the analog signal from this received signal to eliminate the echo and output a received signal that does not include echo. This signal is input to the equalization amount 5, where waveform distortion due to the characteristics of the transmission path is equalized, and a correct reception information signal, which is the same as the transmission signal from the other party, is output to the reception terminal 2.

(Problems to be Solved by the Invention) In the conventional subscriber line transmission device shown in FIG. In the output of , the echo had to be smaller than the allowable residual amount in all time intervals.

This is because the echo canceller 6 does not know where the data identification point will be.

Therefore, the sampling rate of the zero converter 2 needs to be at least twice the reciprocal of the echo frequency band. In reality, although it is not shown in Figure 2, it is common to insert a band-limiting filter before the echo canceller 6 on the four-wire receiving side, so the reciprocal of the frequency band that is considered to be sufficiently attenuated by this filter It needs to be more than twice that. Typically, this will be on the order of four times the transmitted symbol rate. Therefore, in the conventional subscriber line transmission device, there was a problem that the order of the echo filter 8 became extremely large, and the circuit scale of the echo canceller 6 became extremely large.The present invention solves this problem, The purpose of the present invention is to provide a subscriber line transmission device with a small circuit scale.

(Means for Solving the Problems) In order to solve the above problems, the present invention provides a subscriber line transmission device using a hybrid circuit with an equalization amount on the four-wire receiving side of the hybrid circuit. An echo canceller is provided after the equalization amount of , and further, the echo canceler of and is configured to generate an echo component from the transmitted information and subtract this echo component from the output signal of the equalization amount. It is.

(Function) According to the present invention, the subscriber line transmission device is configured as described above, and the echo The data identification point can be known in the canceller. Therefore, in the echo canceller, it is sufficient to suppress the echo at the time of data identification to below the allowable residual amount, and the sampling rate of the A/D converter in the echo canceller is sufficient to be the transmission symbol rate. Therefore, the order of the echo filter in the echo canceller can be small, and the circuit scale can be reduced. Therefore, the above problem can be solved.

(Embodiment) Fig. 1 is a configuration diagram of a subscriber line transmission device showing an embodiment of the present invention. The same elements are given the same reference numerals.

This subscriber line transmission device differs from conventional ones in that the positional relationship between the equalization amount 5 and the echo canceller 6 is reversed.

In the subscriber line transmission device shown in FIG. 1, the echo canceller 6 acts not to cancel the echo itself on the four-wire receiving side, but to cancel the signal when this echo passes through the characteristics of the equalization amount 5. That is, the echo filter 8 approximates the overall characteristics of the echo path from the four-wire transmitting side to the four-wire receiving side and the equalization amount 5. The operation of the other elements is the same as in FIG.

Next, the operation of the subscriber line transmission apparatus of the present invention shown in FIG. 1 will be explained.

A transmission signal inputted from the transmission terminal 1 is inputted from the four-wire transmission side of the hybrid circuit 4, outputted from the two-wire side to the two-wire side terminal 3, and sent out to the transmission path. At this time, this transmission signal is transmitted to the A/D converter 7 in the echo canceller 6.
It is also input to The A/D converter 7 digitalizes the transmitted signal and supplies it to the echo filter 8 . The echo filter 8 approximately generates the same signal as the echo component included in the output of the equalization amount 5 by approximating the overall characteristics of the echo path from the four-wire transmitting side to the four-wire receiving side and the equalization amount 5. and gives this to the D/A converter 9. The D/A converter 9 converts the approximate echo component, which is a digital signal, into an analog signal, and supplies this to the subtraction side input of the subtraction circuit 1θ. On the other hand, a signal from the other party coming from the transmission path is input to the two-wire side of the hybrid circuit 4 via the two-wire side terminal 3, and output as a received signal to the four-wire receiving side. This received signal includes an echo component from the four-wire transmitting side to the four-wire receiving side. This received signal is input to the equalization amount 5, where waveform distortion due to the characteristics of the transmission path is equalized, and the resultant signal is applied to the subtracted side input of the subtraction circuit 10 in the echo canceller 6. The subtraction circuit 10 cancels the echo component by subtracting the approximate echo component converted into the analog signal from the received signal of the equalizer, and the subtraction circuit 10 cancels the echo component by subtracting the approximate echo component converted into the analog signal from the received signal of the equalizer. A correct received information signal, which is the same as the transmitted signal on the side, is output to the receiving terminal 2.

In the subscriber line transmission device of the present invention, the identification clock for determining received data is the output signal of the echo canceller 6, which is the output signal of the subtraction circuit 10, or the equalization amount 5
Since the data is extracted from the output signal of the echo canceller 6, it is possible to know the data identification time point. Therefore, the echo canceller 6 can set the characteristics of the echo filter 8 so as to focus only on the echo component at the time of data identification and suppress it. That is, the echo canceller 6 only needs to cancel the echo component at the time of data identification, and there is no particular need to cancel echoes at other times unless timing jitter is considered. Therefore, the sampling rate of the transducer 7 is sufficient to be the same as the transmission signal rate. Conventionally, sampling was performed at about four times the transmitting signal speed, but with the configuration of the present invention, the sampling speed is about 1/4 of the conventional speed, and the order of the echo filter 8 is also about 1/4. The scale of the echo filter 80 circuit can also be reduced to about 1/4.

As can be seen from the above description, even if the D/A converter 9 is omitted and subtraction is performed using the pulses that constitute the digital signal, there will be no significant effect. In other words, if we look only at the time of data identification, there is no change.

This D/p converter 9 is used to alleviate the deterioration of characteristics due to timing jitter by suppressing echoes around the data identification point to some extent. That is, it performs the effect of waveform smoothing.

FIG. 3 is a block diagram of a subscriber line transmission device using a driver 11 that converts transmission data into a transmission line signal. Elements that are the same as those in FIG. 1 are given the same reference numerals. The only difference from FIG. 1 is this driver 1. In FIG. 1, it can be seen that the output signal of this driver 11 is input to the transmission terminal 1 as a transmission signal.

Since the input signal to the transmission terminal 1 in FIG. 3 is a digital signal, the converter 7 can be a mere sampling circuit, or is unnecessary if the transmission signal can be treated as a digital signal. Therefore, the first
The circuit scale can be made even smaller than shown in the figure. Note that if the A-power converter 7 is not necessary, the sampling operation is also not necessary, but in this case, the clock speed for digital signal processing of the echo filter 8 corresponds to the sampling speed. This makes the transmitted symbol rate sufficient.

(Effects of the Invention) As explained in detail above, according to the present invention, the sampling rate of the converter in the echo canceller is sufficient at the transmission symbol rate, and therefore the order of the echo filter is small. Therefore, the effect of reducing the size of the circuit can be expected.

[Brief explanation of drawings]

FIG. 1 is a block diagram of a subscriber line transmission device showing an embodiment of the present invention, FIG. 2 is a block diagram of a conventional subscriber line transmission device, and FIG. 3 is a block diagram of a subscriber line transmission device showing a second embodiment of the present invention. FIG. 2 is a configuration diagram of a public line transmission device. DESCRIPTION OF SYMBOLS 1... Transmission terminal, 2... Receiving terminal, 3... Two-wire side terminal, 4... Hybrid circuit, 5... Equalization amount, 6
...Echo canceller, 7...Analog/digital converter, 8...Echo filter, 9...Digital/analog converter, 10...Subtraction circuit, 1...
driver. ni L world; I Hatake' I Akanechi) Conventional a'< fr 詑远8 to Figure 2

Claims (1)

[Claims] In a subscriber line transmission device using a hybrid circuit, an equalizer is provided on the four-wire receiving side of the hybrid circuit, an echo canceller is provided behind the equalizer, and the echo canceler is configured to: A subscriber line transmission device characterized in that it is configured to generate an echo component from transmitted information and to subtract this echo component from the output signal of the equalizer.